Bryan Mitchell presented the following explanation :
In view of my historic , and generally lost battles with PTFE tape, I am now
contemplating the use of LS-X compound with my new radiator valves, and I
would value comment on this please.

Plumber's Mate was suggested to me, as able to seal the impossible. I
spent two days struggling to get a seal on two wash basin drains, I
tried PTFE, Boss White - neither worked, but PM did first try. Its a
none setting sort of mastic, difficult to get it to stay put on the
thing you are trying to seal, it prefers to pull back off on your
fingers, but if you persist it is squishy enough to seal any gaps in
your application. Unlike Boss White, it doesn't set in the pot.

I used what I bought around three years ago, to TV's all round my
heating circuit. I filled up and tested for leaks today, there were
none.